1. Field of the Invention
The invention relates to a slit nozzle for applying to a substrate liquid high-polymer material, in particular a hot-melt adhesive based on polyurethane, comprising a nozzle body, a supply passage for the liquid material in the nozzle body, a controllable shutoff valve, a spreading chamber adjoining the supply passage and an exit slit connected to the spreading chamber.
2. Description of the Prior Art
A slit nozzle is disclosed in German patent specification 3,541,784 and comprises a nozzle body, a supply passage for the liquid material in the nozzle body, a controllable shut-off valve in the supply passage, a spreading chamber adjoining the supply passage in the flow direction and an exit slot connected to the spreading chamber so that a closed layer of liquid high-polymer material can be applied to the substrate to be coated, usually a web moving past and beneath the nozzle body.
With this slit nozzle difficulties occur when, during intermittent operation, high switching frequencies have to be reached because then the desired satisfactory cut-off at the exit nozzle is no longer ensured. The material applied to the substrate to be coated then exhibits unpleasant patterns which can lead to problems in further handling.
A further difficulty occurs in the application of hot-melt adhesive based on polyurethane, on contact with water, in particular at high air humidity, such a hot-melt adhesive can react very rapidly ("cracking") so that here very short processing times are desired.